Ultra-short-pulse laser irradiation and ablation of dielectrics

Increasing availability and reliability of ultra-short pulse laser systems is opening up the possibility of using such lasers for ablation and structu ring of dielectrics and other materials (e.g. metals) which cannot be struc tured with high micro-metre precision using standard ns laser systems. In t his chapter the advantage, of visible (800 nm) ultra-short laser pulses for micro-structuring of high band gap materials is discussed. Different pheno mena are observed, related to three fluence regimes. (i) Fluence " damage t hreshold fluence (F-th) In this regime it is possible to produce long chann els with a high aspect ratio and little residual damage and stress in the m aterial. (ii) Fluence = F-th Here, two distinct ablation phases can be obse rved. A "gentle" and a "strong" ablation phase. The gentle phase leads to c ontrolled melting and vaporisation of material, sometimes accompanied by ri pple formation on the surface. At higher laser intensities (or after a suff icient number of incubation pulses) the strong ablation phase is observed w hich we relate to phase explosion. (iii) Fluence < F-th Self-focusing of th e laser light due to the Ken effect can occur in the dielectric materials f or ps pulses. This leads to the possibility of direct writing of micro-stru ctures in the material bulk without producing damage on the entrance or exi t surfaces. Manipulating the laser pulse width and/or pulse energy can cont rol the position of the structures caused in the bulk. Similar non-linear o ptical effects are responsible for material removal from the exit surface o f the material to leave extremely smooth well-defined micro-metre sized con e-shaped holes.